1. Technical Field
This invention relates to gas turbine engines, and in particular to turbofan engines.
2. Background Information
Minimizing the noise generated by gas turbine engines, such as are used in aircraft, without significantly increasing the cost, complexity, structural integrity and performance of the engine is an important consideration in engine design. Some turbofan engines, especially high bypass ratio turbofans, are known to generate significant noise as a result of unsteady flow within the fan stage between the fan rotor and the immediately following row of fan exit guide vanes (FEGV""s) in the bypass duct.
One technique that has been demonstrated to successfully reduce that noise is to sweep (i.e. tilt) the FEGV""s rearwardly across the duct from their radially inner to their radially outer ends. This has the effect of increasing the spanwise phase variation of the rotor wake interaction with the stator vane, causing more noise cancellation. Significant reductions in rotor-stator interaction tone noise have been demonstrated experimentally with this swept stator vane design concept, as discussed in xe2x80x9cDesign Selection and Analysis of a Swept and Leaned Stator Conceptxe2x80x9d, by E. Envia and M. Nallasamy, Journal of Sound and Vibration, Vol. 228, 1999, pp 793-836. Additional benefit has been realized in terms of fan broadband noise reduction with swept stator vanes. The mechanism of broadband noise reduction associated with swept stator vanes is not due to spanwise phase cancellation. Rather, it is thought to derive from a reduction in the normal component of velocity incident upon the stator vanes. The idea is to reduce the normal component of the fan flow against the leading edge of the vane. It is that normal component which is believed to create a major portion of the noise. This is discussed in a paper by Donald B. Hanson, xe2x80x9cInfluence of Lean and Sweep on Noise of Cascades with Turbulent Inflowxe2x80x9d, AIAA 99-1863, AIAA/CEAS 5th Aeroacoustics Conference, Seattle Wash., May 10-12, 1999.
FIG. 1 is representative of the prior art. In FIG. 1, which is a simplified cross section of the forward portion of a gas turbine engine 10, a plurality of circumferentially spaced apart fan blades 100 rotate around the engine centerline 102. Airflow entering the engine inlet 103 passes through the row of fan rotor blades and is split between the core engine flow path 104 and the fan bypass flow path 106. The core flow passes through multiple stages of the compressor 108. The bypass flow passes through a row of circumferentially spaced apart FEGV""s 110 in the bypass flow path. The FEGV""s remove the swirl imparted to the bypass flow by the fan blades 100 and redirect the flow substantially axially. In accordance with the prior art, to reduce noise, the leading edges 112 of the vanes 110 are swept rearwardly, across the full radial extent of the bypass duct, from their radially innermost ends 114 to their radially outermost ends 116.
One drawback of using swept FEGV""s of the type described in the prior art is the additional axial length required within the bypass duct to accommodate the sweep, as compared to vanes with substantially radially extending leading and trailing edges. Furthermore, swept vanes of the prior art are longer, and thus heavier and costlier, than unswept vanes. For these reasons it is desirable to find alternative means to reduce the noise hereinabove described.
According to the present invention, the fan exit guide vanes within the bypass duct of a turbofan engine each have their leading edges extending across the fan bypass duct from duct inner wall to the duct outer wall, each leading edge including (a) a radially inner portion extending outwardly from the inner wall and having a sweep angle A, and (b) a radially outermost portion extending inwardly from the outer wall for 5% to 50% of the span of said airfoil and having either a rearward or forward sweep angle B of between 10xc2x0 and 60xc2x0, wherein if sweep angle B is forward, then sweep angle A is a rearward sweep angle or 0xc2x0, and if sweep angle B is rearward, then sweep angle A is forward or 0xc2x0.
As used in this application and the appended claims, a vane sweep angle B is the acute angle formed between the portion of the vane leading edge where sweep is to be determined and a radial line passing through that leading edge portion. (Note: The Hanson reference mentioned above defines two conventions, a xe2x80x9ccascade systemxe2x80x9d convention and a xe2x80x9cduct systemxe2x80x9d convention to describe lean and sweep geometry. The sweep angles A and B defined in this application are based upon the cascade system convention.) In cases where the leading edge portion is not simply a straight line, for purposes of determining the sweep angle, replace the leading edge line of that portion with a closest fit straight line. If the leading edge portion is parallel to the radial line, the sweep angle is 0xc2x0. A leading edge with a xe2x80x9cforwardxe2x80x9d sweep angle extends radially outwardly from a downstream point to an upstream point. A leading edge with a xe2x80x9crearwardxe2x80x9d sweep angle extends radially outwardly from an upstream point to a downstream point. Throughout this application and appended claims, the sweep angle xe2x80x9cAxe2x80x9d represents the sweep angle of the leading edge of a radially inner portion of the vane span adjacent the inner flow path wall, and a sweep angle xe2x80x9cBxe2x80x9d represents the sweep angle of the leading edge of the radially outer portion of the vane span adjacent the outer flow path wall.
As mentioned above, some of the noise created by the interaction of the fan flow and the FEGV""s may be reduced by reducing the component of the fan flow normal to the FEGV leading edge. It is for this reason that swept vanes have been used successfully in the prior art. The present invention is based upon experimental rig data and mathematical modeling that indicates a very significant portion of the noise is broadband noise created by high levels of inflow turbulence at the radially outermost portion of the span of the FEGV""s, especially the outermost 10% to 30% of the span. While it may be beneficial to sweep only the outermost 5% of the span, sweeping more than 50% of the radially outermost portion of the span adds axial length and weight to the vane row without necessarily providing worthwhile additional broadband noise reduction.
The sweep may be either forward or rearward, as further explained in the Detailed Description of the Present Invention, below. Because the sweep is preferably applied only to the radially outermost portion of the vanes, the overall axial length of each vane is considerably less than the length of prior art swept vanes, which are swept in one direction over the entire vane span. Because the swept vanes of the present invention do not require as much flow path axial length as swept vanes of the prior art having the same sweep angle, they may use a greater sweep angle than used in prior art and still provide a benefit of less flow path axial length and less weight.
It is also contemplated that, in addition to sweeping the radially outer portion of the FEGV""s, the radially inner portion of the FEGV""s may also be swept, but in the opposite direction. This could be done if it were determined that sweeping the radially inner portion of the vanes added additional noise benefits that outweighed other considerations, such as performance and cost penalties. Sweeping the radially inner portion of the FEGV""s, or some part of the inner portion, such as that portion adjacent the inner bypass duct wall, in conjunction with but opposite to the sweep direction of the outer portion, could be done without any increase in the axial length of the vane row.
The foregoing features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof as illustrated in the accompanying drawings.